Weed management in rice-based cropping systems in Africa.
Weed competition is a major constraint in all the rice production systems in Africa. In addition to the costs of weed control, weeds account for yield losses estimated to be at least 2.2 million tons per year in sub-Saharan Africa, valued at $1.45 billion, and equating to approximately half the current total imports of rice to this region. Important weeds in upland rice include the perennial species Cyperus rotundus, Imperata cylindrica and Chromolaena odorata, the annual species Euphorbia heterophylla, Digitaria horizontalis, and the parasitic weeds Striga spp. In lowland rice the perennial weeds: Cyperus rotundus, C. esculentus and Oryza longistaminata and annual weeds Sphenoclea zeylanica, Echinochloa spp., Cyperus difformis, C. iria, Fimbristylis littoralis, Ischaemum rugosum, and O. barthii cause serious losses. Common weed management practices in rice-based cropping systems include soil tillage, clearance by fire, hand- or hoe-weeding, herbicides, flooding, fallow and crop rotations, and these are often used in combination. Labor shortages and lack of access to information, inputs, and credits are widespread constraints for African farmers. To optimize financial, social and environmental costs and benefits, integrated and ecological management approaches are advocated. Locally adapted and affordable combinations of preventive measures and interventions should be targeted. Future weed research should aim to deliver the information and tools for the implementation of these approaches. This requires the generation of knowledge on weed biology and ecology and on the consequences of changes in management and the environment on weed populations. To address the diversity of rice-based cropping systems in Africa, priorities need to be set and products and information delivered that take full account of local conditions. This will require farmer participatory approaches that are inclusive with respect to resource-poor farmers and gender.